1. Field of the Invention
The present invention relates to a vacuum valve bellows used in a vacuum valve to separate a driving part for driving a valve member from a main flow path disposed in a vacuum atmosphere.
2. Description of the Related Art
Conventionally, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-340344, a vacuum valve in which a driving part of a valve member is separated from a main flow path disposed in a vacuum atmosphere by a molded bellows is known as a high-vacuum L valve. This is substantially the same as a vacuum valve illustrated in FIG. 3, except for the detailed configuration of the bellows.
The configuration will be described in more detail. This vacuum valve includes a first main port connected to one of a vacuum pump and a vacuum chamber, a second main port connected to the other of them, a ring-shaped valve seat formed in a main flow path connecting the above-mentioned main ports in a housing, a disc-shaped valve member that comes into contact with or moves away from the valve seat, thereby opening or closing the main flow path, and a driving part for axially driving a rod connected to the valve member, causing the valve member to perform opening and closing motions. To separate the driving part of the valve member from the main flow path in the housing disposed in a vacuum atmosphere, an expandable and contractible molded bellows is provided between the circumference of the disc-shaped valve member and the housing so as to surround the rod.
Typically, the above-described known vacuum valve bellows is fabricated by hydrostatic bulge forming, in which multiple ring-shaped molds are arranged at equal intervals around an untreated pipe composed of a metal pipe and sealed at both ends in a liquid-tight manner, a high-pressure liquid is introduced into the untreated pipe, causing portions of the outer periphery of the untreated pipe that are not in contact with the ring-shaped molds to swell outward, and then the distance between the multiple ring-shaped molds is reduced, and the portions of the untreated pipe swelled outward in a ring-shaped shape serves as peak portions, and ring-shaped portions that are not swelled serves as valley portions.
Typically, this molded bellows is formed such that the radius of curvature of the peak portions in cross section (peak R) is equal to the radius of curvature of the valley portions in cross section (valley R). When the molded bellows is attached to a vacuum valve and used, as the bellows is expanded and contracted, the stress acting on the valley portions tends to cause a fatigue fracture compared with the peak portions. Thus, usually, a crack is generated at the middle portion of the side surface of a valley portion of the bellows (part a in FIG. 1), in the process leading to the end of life.